Highly Stereoselective Alkynylation with Steroidal Alkyne: Efficient Pathway to 24(S) or 24(R)-Hydroxylcholesterol

24(S)- or 24(R)-Hydroxyl cholesterol was prepared in high stereoselectivity (up to 99% de), utilizing asymmetric alkynylation of isobutyraldehyde with steroidal alkyne catalyzed by Zn(OTf)2 respectively in the presence of (1S, 2S)-3-(t-butyldimethylsilyloxyl)-2-N, N-dimethylamino-1-(p-nitrophenyl)- propane-1-ol or its (1R, 2R)-enantiomer.